The present invention relates to apparatus for testing electronic circuit boards and other electronic assemblies. Traditional test for such boards use power or timing values corresponding to worst case conditions. Still other tests utilize a variety of patterns to eliminate those products that are pattern sensitive. Varying the input to such circuit boards/assemblies and particularly to digital boards provides an excellent means for finding bad or marginal parts and inefficient or improper manufacturing processes. Such test have done little to identify parts that are vulnerable to the so-called infant mortality failure mode.
Environmental stress screening is a process used to eliminate part and workmanship defects during the manufacture of electronic equipment. Such screening has been defined as the process or method where one hundred percent of a lot or group of identical items is subject to the application of physical climatic stresses or forces (or combinations thereof) to identify and eliminate defective, abnormal or marginal parts in manufacturing defects. The principle objective of environmental stress screening is to transform latent part and workmanship defects (inherent weaknesses that have a high probability of resulting in an early life failure) into detectable failures. This allows for their elimination before delivery to the end-users of the equipment. In other words the thrust of electronic stress screening is to find defects and to force latent defects to turn into failures prior to shipment. It has been established that failures that would not arise during the life of a device, under normal conditions, until 300-240,000 hours of operation, can be forced to occur in up to 48 hours by performing environmental stress screening. Thus, the initiation of failure by environmental stress screening results in a better product which will have fewer field failures. Simultaneous electronic testing with a load is essential during environmental stress screening because many failures are temperature dependent, that is, they will only show up at elevated or lowered temperatures.
Many leading corporations in the computer, telecommunication, automotive, military and aerospace segments of the electronics industry are currently employing environmental stress screening instead of or in conjunction with burn-in (elevated temperature only operation for extended periods). The screening parameters include variations in environmental conditions including dwell temperatures, dwell times, transition rate, number of cycles, etc.
Various apparatus has been devised for performing such tests. A typical arrangement is to provide a chamber with a printed circuit board that extends through a wall of the chamber. The board has an edge connector on each side of the wall. One edge connector, located on the outside of the chamber, is coupled to a load board. The other edge connector, located inside the chamber, is connected to the printed circuit board to be tested either directly or indirectly by means of a product carrier adapter. The product carrier adapter, in some prior art apparatus, is a module into which the printed circuit boards to be tested are mounted.
It is of great importance to have good electrical connections between the load module and the boards being tested to insure not only that any malfunctions are detected but also to insure that the board being tested is being subjected to the desired test conditions. Conventional connectors have, at least in some cases, not provided the desired degree of reliability. Conventional connectors which work well in the ambient environments, often cannot withstand the rigors of repeated environmental stress screening since the process causes them to repeatedly expand and contract. Still another problem is that a typical test arrangement will have many connections. For example, in the apparatus of the present invention there might be 128 connectors which each have 100 individual connections, thus, there would be 12,800 connections.
With a conventional insertion force of one quarter of one pound per individual connection, the total insertion force exceeds 1.5 tons! There is no other known apparatus that actually uses a such a large number of connectors, in part, because of the large forces and the danger of damage to the boards and connectors if there is even slight misalignment.
A limitation on the speed with which a product carrier pallet may be installed and removed from such environmental chambers and thus the utilization of such chambers is dependent on the speed with which the edge or other connectors joining the product carrier pallet to the load module can be accomplished.
Zero insertion force connectors are known. These connectors are typically operated mechanically or pneumatically. A problem inherent in attempting to use such connectors in conventional chamber structures is that the zero insertion force connector cannot tolerate the environmental conditions which are present in the chamber.
It is an object of the present invention to provide apparatus which will permit more rapid removal and installation of product carrier pallets in such chambers.
Another object of the invention to provide apparatus which utilizes zero insertion force connectors and which avoids degradation of the zero insertion force connectors due to the environmental conditions present in the chamber.
Still another object of the invention to provide apparatus which will utilize connectors having a high reliability to insure that the operating testing to be conducted is properly performed and properly monitored.
Yet another object of the invention to provide apparatus which can be rapidly reconfigured for different applications.
Another object of the invention is to provide apparatus which is readily adapted to either partial or complete automation.
Still another object of the invention is to provide some embodiments of the present invention which will more efficiently utilize the interior volume of anenvironmental stress screening chamber particularly where the product to be tested is relatively short as compared to the depth of the environmental stress screening chamber being utilized.